Such a flow guide and such a device are known from Dutch Patent Application NL-A-9400445. The turbine wheel gas meter of this known device consists of a circular symmetrical element which is mounted in the pipe by means of spokes. The gas flow is guided along the upstream blunt, rounded nose part of the turbine wheel gas meter to the annular gap between the central element and the inside wall of the flow pipe. A blade wheel is mounted in the turbine wheel gas meter in such a way that the blades are situated in the annular gap. The blade wheel is driven by the gas flowing through the annular gap. The flow guide is placed at a distance upstream of the nose part, which flow guide in the case of this known device consists of a plate in which a circular symmetrical hole pattern has been made. The flow guide eliminates the eddies in the gas flow. These eddies occur when there are obstacles in the gas pipe, for example at bends, transitions to other pipe diameters or at fittings. After passing through the flow guide, the gas flow is homogenized by radial impulse exchange in the space between the flow guide and the nose part, so that a homogenous or uniform flow profile is produced in the annular gap between the pipe wall and the turbine wheel gas meter. This homogenous flow profile is necessary to permit accurate measurements to be carried out by means of the turbine wheel gas meter. For that purpose, the nose part must be a rounded, slightly blunt shape which is not streamlined, so that sufficient impulse exchange with the gas flow occurs.
However, the pressure loss in the case of this known device is relatively great. This is a disadvantage in particular in circumstances in which not much pressure is available, as in the case of low-pressure systems. This pressure loss is also a disadvantage if the gas is being compressed again after it passes through the turbine wheel gas meter. The compression costs are higher because of the pressure loss which has occurred.
Other known flow guides were designed in the first instance for use in the case of orifice meters instead of in turbine wheel gas meters. For this, the flow guide not only has to eliminate eddies, but also to give a flow profile which within certain margins of error corresponds to a fully developed flow profile such as that which would occur at the end of a very long pipeline. The flow profile needed for an orifice meter has to be a very specific shape. The flow guides needed for this of necessity produce a relatively high pressure loss.